It is a known technique to arrange, on the entry or feed side of a cutting machine, in front of the front saw blades, lateral guiding devices which carry the as yet uncut piece of timber towards the saw teeth in the required position. An example of a cutting machine adapted for use with the inventive guiding mechanism is described in U.S. Pat. No. 3,692,074. These guiding devices may, for example, be made up of a guide mechanism which comprises a numer of pairs of rollers arranged behind each other in the direction of feeding of the piece of timber. The spacing between the rollers is adjustable, and is increased to receive each new piece of timber. The distance is then reduced so that the piece of timber is firmly enclosed by the rollers on both sides. In the case of a reducing saw, the lateral guiding devices are formed partly by the reducing discs themselves, and partly by flat guide plates adjacent to the reducing discs. Here too, the distance between the lateral guiding devices at right angles to the direction of feeding is controllable.
When the rear end section of the piece of timber, usually the root end section, has left these guiding devices arranged in front of the saw blades, the piece of timber is no longer laterally guided. Only at a certain distance after the saw blades are there lateral guide rollers arranged on the output side. Thus the piece of timber is not laterally guided at all at the place where it is being cut. Problems arise when the rear end face of the piece of timber has passed the last lateral support on the feed side. Although the distance to be covered to the saw teeth is relatively short (of the order of 50 mm), the rear end section of a curved piece of timber may nevertheless have an adverse effect on the saw blade or blades, such as by applying a lateral twist to the blade or blades. This effect results because the cut front section of the piece of timber being fed out of the machine is laterally gripped in the guide rollers on the output side, which, depending the extent to which the piece of timber has a curved plan profile shape, means that, when all support on the feed side has ceased, a torque is produced with its centre of rotation at the sawing point, i.e. acting directly on the saw blades. In sawing machines with more than two saw blades arranged at two different distances from the feed end, the distance to the more remote saw blades is even greater, and the torque acts on these saw blades for an even longer time.
The pieces of timber are fed into the above described cutting machines continuously, one after the other. The distance between the rear end face of one piece of timber and the front end face of the next piece of timber is known as the "log gap". When the log gap is shorter than the length of the guide mechanism (the shorter the log gap, the more economical the process), the guide mechanism must be open to receive the next piece of timber even before the preceding piece of timber has completely left the guide mechanism, which means that approximately 300 or 600-700 mm (with respect to the more remote saw blades) of the end section of the piece of timber is unguided. In the quality control of sawn timber, the thickness is customarily measured within, among others, the first "module" (3 dm) from each end face, meaning that one of these quality control measurements is taken within the above-mentioned rear end section of 300-700 mm which is unguided.